Why is Full-speed USB considered differential and what is its common mode?

I am reading USB 2.0 standard, particularly about High-speed part, and I can't understand why do we consider that HIgh-speed USB uses differential signaling. Or at least why it is balanced? For transmitting logic 0 or 1, USB transmitter drives either one line or another with 17.7mA, and if said differential, I would expect currents with the same intensity but opposite directions. Because of my previous problems to understand this issue, i am not able to understand what is than a common mode on the USB bus in High-speed mode.